TIME -- It was a thorny debate — but, it turns out, the question of what it means to be progressive in America is even more complicated than that. “Historians don’t have a definition [of ‘progressive’] and we’ve argued for years over whether there is such a thing,” says Charles Postel, an expert on the Progressive and Populist movements who teaches history at San Francisco State University. First of all, Postel says, it’s worth noting that today’s politicians who claim the progressive mantel do share a lot with their forebears. Sanders’ respect for Eugene Debs, the socialist leader of the Progressive Era, who believed that the Progressive Party had stolen the Socialist platform, is well known. As for Clinton, the historian sees a strong connection between the political stances and personal lives of the candidate and Jane Addams, one of the foremost female activists of the Progressive Era.